The present invention relates to a protection circuit for a low noise amplifier used, for example, in a receiver on the ground for receiving at the ground microwave signals from communication satellites, broadcast satellites, etc.
With the quick development of information network systems, the demand of satellite communication systems is sharply increasing and their frequency bands are going toward higher frequencies. As high frequency field effect transistors, Schottky barrier type field effect transistors (MESFET: MEtal-Semiconductor junction FET) using compound semiconductors such as GaAs, are put to practical use. In addition, in order to realize smaller, lower cost and higher performance systems, the first amplification stage of the down converter circuit, where high frequency signal is converted to low frequencies, has recently designed as an MMIC (Microwave Monolithic Integrated Circuit).
GaAs-used FETs are superior devices, having low noises even at very high frequencies such as microwave bands, but they have a defect to be weak in electrostatic breakdown as compared with silicon devices. For a long time, efforts have been made to protect a circuit from such electrostatic breakdown. For a low noise amplifier, however, its noise figure is greatly governed by the noise figure of the input circuit. Therefore, there is no practical low noise amplifier with a protection circuit capability because adding an extra circuit to the input circuit may deteriorate the noise figure.
Moreover, it has so far been usual that such a low noise amplifier is connected to a parabolic antenna whose primary radiator mostly has a wave guide structure. Therefore, it has been possible to prevent the electrostatic breakdown only by carefully treating the low noise amplifier (or the circuit including it). This may be one reason why this kind of study has not been so active.
Meanwhile, flat antennas have begun to be put to practical use as ground antennas to receive microwave signals from communication satellites or broadcast satellites. A flat antenna, consisting of many antenna elements located in a flat plane, gathers signal power from each element via conductive wire and provides them as a single output. In the early stage, microwave reception flat antennas were greatly inferior to parabolic antennas both in cost and performance. These days, however, they have reached the level of practical use by flourishing study of micro strip antennas since the late 1970s as well as the improved performance of microwave-use printed circuit boards.
It is also expected that the advance of MMIC (Monolithic Microwave Integrated Circuit) technology will promote the spread of flat antennas because smaller antennas are preferred.
On the contrary, how to connect a flat antenna to a reception system has not necessarily been studied sufficiently. For example, there is a method based on a conventional technology in which a wave guide, which is an ordinary microwave propagation means, is used as a means for connecting them both. However, this method makes it difficult to achieve total miniaturization and weight reduction. Thus, advantages of the miniaturization of the reception systems and the flat shape of the antennas are not sufficiently utilized.
On the other hand, connecting a flat antenna directly to a reception system is difficult. This is because a flat antenna has an antenna pattern (metal) printed film sandwiched between insulators. Thus, the antenna may be charged greatly enough to damage the reception system. The present invention intends to solve such a problem.